An abdominal aortic aneurysm (AAA) is an abnormal swelling of the lower part of the aorta that extends through the abdominal area. The aorta is the primary blood vessel that transports blood from the heart to the rest of the body. The walls of aorta are elastic, which allow the vessel to be filled with blood under high pressure. An aneurysm occurs when the arterial walls become weakened and distended. Many factors can contribute to the weakening of arterial walls, including atherosclerosis, high cholesterol, hypertension, and smoking.
An aneurysm that has become too large may rupture, which is extremely dangerous. Symptoms of a ruptured aneurysm include excruciating pain of the lower back, flank, abdomen and groin. Bleeding associated with the rupture often leads to hypovolemic shock, and if left untreated, will result in a relatively quick death.
Conventional methods of repairing abdominal aortic aneurysms include surgical intervention and minimally invasive procedures like endovascular aneurysm repair (EVAR) and thoracic endovascular aneurysm repair (TEVAR). In the EVAR procedure, a stent graft (also known as an “endograft”) is generally inserted into the aorta through small incisions in the groin. The stent graft reinforces the weakened part of the vessel from the inside and creates a new channel through which the blood flows, eliminating the risk of rupture at the site of the aneurysm. A primary concern associated with EVAR is that, despite placement of the stent graft, blood may continue to flow into the aneurysm, in what is commonly known as an endoleak. Endoleaks arising after grafting may be attributed to an incomplete sealing between the stent graft and the aortic wall or defects within the stent graft itself. Endoleaks are the major cause of complications in EVAR and TEVAR procedures, and thus failure in endoluminal treatment of AAA. When an endoleak occurs, it causes continued pressurization of the aneurysm sac and may leave the patient at risk of an AAA rupture and subsequently, immediate death.
Endoleaks are classified based on the origin of blood-leakage. There are generally five types of endoleaks (Type I through Type V). A type I endoleak is a perigraft leakage at proximal or distal stent graft attachment sites (near the renal and iliac arteries), a type II endoleak is a retrograde flow from collateral branches, such as the lumbar and inferior mesenteric arteries. A Type III endoleak is a leakage between overlapping parts of the stent (i.e. connection between overlapping components) or rupture through graft material and a type IV endoleak is leakage through the graft wall, generally due to the quality (porosity) of the graft material. A type V endoleak is generally expansion of the aneurysm sac without an identifiable leak (also called “endotension”).
Type I and Type III leaks are considered to have high risk to the patient and must be identified and fixed during an EVAR/TEVAR procedure. A type I endoleak may be due to mal-apposition, or graft enfolding, of the stent graft at the proximal or distal landing zones (i.e., portions of the stent graft are not touching the luminal wall). A type III endoleak may be due to a damaged stent graft (e.g., hole in graft) or misaligned overlapping segments of the graft, causing a hole in the middle, hence, a leak. If either, or both, of these types of endoleaks are present, the aneurysm will continue to fill with blood and experience high pressure, leading to a high risk of rupture to the patient. As such, the identification and repair of these types of endoleaks is paramount to ensure patient safety and procedural success.
Currently, during EVAR/TEVAR procedures, physicians attempt to detect such leaks with existing imaging techniques, such as external imaging modalities (e.g., angiography, fluoroscopy, computed tomography (CT), and magnetic resonance imaging (MRI)). Based on the images, a physician may repair the leaks by correcting the graft-deployment, so as to ensure an appropriate seal. However, the use of non-invasive imaging techniques is restricted to either pre-procedural planning (CT and MRI) or peri-procedural monitoring with known limitations of the incidence-angle (angiography). Additionally, the use of external imaging techniques may be limiting and fail to provide the level of detail that intraluminal imaging techniques are able to offer. As such, the level of detection and monitoring of endoleaks may be sacrificed.